1. Field of The Invention
The present invention relates to an antenna for transmitting and/or receiving electromagnetic radiation, and more particularly to an antenna which is suitable to be used for portable radio equipment.
2. Description of The Prior Art
In recent times there has been significant development in portable radio equipment such as paging systems and land mobile radio systems, etc. With the advances of technologies in this field, demand for small antennas which are suitable to be used for such equipment has been increasing. In order to design the antenna for portable radio equipment, four factors given below are the important factors which should be taken into account.
(1) Little degradation of the input impedance and gain characteristic when the antenna is placed near an electric circuit and a human body;
(2) Good electrical isolation between the antenna and the ground circuit of a transmission line or an electric circuit so that the antenna current should not flow on the ground circuit and the case of the equipment;
(3) High gain and omnidirectional radiation pattern in the horizontal plane; and
(4) Small size, light weight and firm structure.
Among these factors, factor (1) is particularly important in the case when the antenna is to be used as a build-in type.
External sleeve antenna are usually used with portable radio equipment. This kind of antenna is disclosed in S. A. Schelkunoff, H. T. friis: "Antennas Theory and Practice" John Wiley & Sons (1952). The sleeve antenna is featured by its good electrical isolation between the antenna and the ground circuit of a coaxial transmission line and an electric circuit, where the coaxial line is used to convey energy from the transmitter to the antenna or from the antenna to the receiver. A quater-wave trap, which is often called "balun" or "Sperrtopf", is used at a feed point of this kind of antenna. The sleeve antenna can be considered as a modification of a simple quater-wave monopole antenna, so that the parastic current on the outer surface of the outer conductor of the coaxial transmission line is reduced or eliminated by means of a quater-wave trap. Due to the above unique characteristics, the sleeve antenna shows fairly good performance as an external antenna for portable radio equipment. However, the antenna has to be more than one-half wavelength long, and the input impedance and gain characteristic of the antenna are easy to degrade due to access of an electric circuit and a human body to the antenna. Therefore, the sleeve antenna is not suitable as a build-in antenna for portable radio equipment. On the other hand, an antenna having a microstrip configulation is very attractive as a build-in antenna for portable radio equipment, because it is very small in size, simple form of low-plofile in shape and firm in structure. This kind of antenna is disclosed in IEEE Transactions on Antenna and Propagation, vol. AP-29, No. 1, pp. 1-183, January 1981. In this article, FIG. 5 on page 6 shows a basic structure of a rectangular microstrip antenna. This microstrip antenna has a sandwitch structure of two parallel conducting layers separated by a single thin dielectric substrate. The lower conductor functions as a ground plane, and the upper conductor may be a simple resonant rectangular patch. The ground plane is considered as a electrically conducting plate which is extended in X-Y plane infinitely or which has a large size relative to the wavelength of signal. As an antenna for a portable radio equipment, the ground plane has to be practically as small as possible, and may be required to have almost the same size as the resonant rectangular patch. In this situation, however, the ground element no longer acts as the "ground" on which a constant potential voltage should be maintained, but a sinusoidal variation of a voltage distribution is produced on the ground plane. Therefore, if a coaxial transmission line is used to transfer signals between the antenna and the equipment, a parastic current is generally induced on the outer conductor of the coaxial transmission line. Under this condition, the transmission line acts as a part of antenna element. As a result, the characteristics of the antenna such as the input impedance, radiation pattern and gain will change easily under actual usage conditions.